An important element you leave out, is that it took a piecemeal collection of agencies and entities to get all of Trumps information.
Trump took every effort to get his tax and business records all the way to the Supreme Court (a delay tactic) so that it took one entity two years to get his tax records, and another two years to get his business records. And only by cooperating with each other, could they compare the two.
And when they did, is when they found the multiple crimes that Trump committed.
Which of course explains why Trump spent so much time and effort to keep his tax returns away from congress, away from US Attorneys, away from District Attorneys, and away from Attorney Generals.